A typical example of such a charging circuit is a photoflash charging circuit that charges a capacitor used for turning on a photoflash lamp. FIG. 7 shows the circuit configuration of a conventional photoflash charging circuit.
In this photoflash charging circuit, battery 12, switching element 14, and resistor 16 for current detection are connected in series on the primary side of transformer 10. Diode 18 for rectification and capacitor 20 for storing electric power are connected in series on the secondary side of transformer 10. Photoflash lamp 22 connected to capacitor 20 on the secondary side is constituted by, for example, a xenon flash lamp. It is normally kept in an insulated state and instantaneously discharges (becomes conductive) when a high voltage is applied from a trigger circuit (not shown in the figure) to trigger electrode 23 when taking a picture. When lamp 22 discharges, the electric power stored in capacitor 20 flows into lamp 22, and the xenon gas in the lamp becomes luminescent.
Battery 12 is connected between one terminal 24a of primary coil 24 and ground potential as the reference potential on the primary side of transformer 10. Switching element 14 and resistor 16 are connected in series between the other terminal 24b of primary coil 24 and the ground potential. When switching element 14 is on, current (primary current) Isw flows in the closed loop on the primary side, and electromagnetic energy is stored in primary coil 24. When switching element 14 is turned off, the primary current Isw that has been flowing is cut off. At the same time, the electromagnetic energy stored in primary coil 24 is transferred to secondary coil 26 through electromagnetic induction. Current (secondary current) Iout flows in the forward direction through diode 18 in the secondary circuit. Capacitor 20 is charged by said secondary current Iout. When switching element 14 is turned on/off repeatedly, the charging voltage Vout of capacitor 20 can be increased stepwise.
On the primary side of the photoflash circuit, comparator 28 is used to obtain the timing of switching element 14 from the ON state to the OFF state, and a comparator 30 is used to obtain the timing of switching element 14 from the OFF state to ON state.
The two terminals of resistor 16 are connected to the two input terminals (+), (−) of comparator 28. More specifically, the positive terminal of resistor 16 is directly connected to the positive input terminal (+) of comparator 28, and the negative terminal of resistor 16 is connected to the negative input terminal (−) of comparator 28 via the ground and reference voltage source 32. If the resistance of resistor 16 is R16, the voltage drop V16 is V16=Isw·R16. If the voltage of reference voltage source 32 is taken as VF1 (constant value), the output signal U1 of comparator 28 is at level L when V16<VF1. The output signal of comparator 28 changes from level L to level H when V16≧VF1. The output terminal of comparator 28 is connected to the reset input terminal (R) of RS latch circuit 34 constituted by an RS flip-flop. The output terminal (Q) of RS latch circuit 34 is connected to the control terminal of switching element 14. When the output signal U1 of comparator 28 switches from level L to level H, RS latch circuit 34 is reset at the rising edge of the output signal. The output signal of RS latch circuit 34, that is, the switching control signal Csw, switches from level H to level L. As a result, switching element 14 is switched from the ON state to the OFF state.
The two terminals 24a, 24b of primary coil 24 are connected to the two input terminals (+), (−) of comparator 30. More specifically, one terminal 24a (on the side of battery 12) of primary coil 24 is directly connected to the positive input terminal (+) of comparator 30. The other terminal 24b (on the side of switching element 14) of primary coil 24 is connected to the negative input terminal (−) of comparator 30 via reference voltage source 36. During the period when primary current Isw flows in the primary circuit, since the voltage on the side of terminal 24a of primary coil 24 (output voltage VBAT of battery 12) is not lower than the voltage on the side of terminal 24b of primary coil 24 (terminal voltage VSW of switching element 14), the output signal U2 of comparator 30 becomes level H. When switching element 14 is switched from the ON state to the OFF state and primary current Isw stops instantaneously, an induced electromotive force, with terminal 24b as the positive side and terminal 24a as the negative side, is generated between the two terminals of primary coil 24 as a result of electromagnetic induction. An induced electromotive force or secondary voltage VA, obtained by multiplying the winding ratio by the induced electromotive force on the primary side, is also generated in secondary coil 26. In this case, the mutual inductance of transformer 10 is negative, and primary coil 24 and secondary coil 26 have opposite polarities. In other words, terminal 26a of secondary coil 26 with the same polarity as terminal 24a of primary coil 24 is connected to ground. Terminal 26b of secondary coil 26 with the same polarity as terminal 24b of primary coil 24 is connected to the anode terminal of diode 18. Consequently, the induced electromotive force or secondary voltage VA generated between the two terminals of secondary coil 26 has terminal 26b as the positive side and terminal 26a as the negative side, and thus becomes a bias in the forward direction of diode 18. Secondary current Iout flows in the secondary circuit to charge capacitor 22.
On the other hand, when an induced electromotive force, with terminal 24b as the positive side and terminal 24a as the negative side, is generated between the two terminals of primary coil 24 as described above, the output signal U2 of comparator 30 changes from level H to level L. The output terminal of comparator 30 is connected to the input terminal of one-shot circuit 38. The output terminal of one-shot circuit 38 is connected to the set input terminal (S) of RS latch circuit 34. One-shot circuit 38 does not operate in response to the dropping of comparator output signal U2 from level H to level L. When secondary current Iout flows in the secondary circuit as described above, the electromagnetic energy of transformer 10 is converted to secondary current Iout and is gradually reduced. The secondary voltage VA gradually drops. The voltage between the two terminals of primary coil 24 also gradually drops in proportion to secondary voltage VA.
When the value obtained after subtracting voltage VF2 (constant value) of reference voltage source 36 from the voltage between the two terminals of primary coil 24 is negative, that is, when the switch terminal voltage VSW is lower than (VBAT+VF2) obtained by adding the voltage VF2 of reference voltage source 36 to the output voltage VBAT of battery 12, the output signal U2 of comparator 30 changes from level L to level H. Correspondingly, a positive pulse G with a certain pulse width is output from one-shot circuit 38, and a set signal is input into the set input terminal (S) of RS latch circuit 34. As a result, the output signal of RS latch circuit 34, that is, switching control signal CSW, switches from level L to level H. As a result, switching element 14 is switched from the OFF state to the ON state. Subsequently, the ON/OFF state of switching element 14 is repeated as the operation and control are repeated. The charging voltage (Vout) of capacitor 20 is increased gradually or stepwise to the set level.
FIG. 8 shows the operation of said photoflash charging circuit (FIG. 7) depending on the signal waveform of each part. In this figure, switching element 14 is switched from the ON state to the OFF state at times t1, t3. Switching element 14 is switched from the OFF state to the ON state at times t2, t4. As shown in the figure, as the charging voltage Vout of capacitor 20 rises, the secondary voltage VA induced on the secondary side rises immediately after switching off, and the negative slope of secondary current Iout is increased (in other words, the flowing period of secondary current Iout becomes shorter). When the charging voltage Vout of the capacitor reaches the set level, secondary voltage VA reaches a prescribed level. Charging end detecting circuit 40 shown in FIG. 7 detects the end of charging of capacitor 20 when the induced electromotive force on the primary side correlated with secondary voltage VA reaches the set value and turns off switching element 14 to stop the charging operation. Then, switching element 14 is kept in the OFF state until the next charging operation is started.
FIG. 9 shows the state transition and its conditions for said photoflash charging circuit (FIG. 7). Mode A, in which switching element 14 is turned on and the power supplied from battery 12 is stored in transformer 10 in the form of electromagnetic energy, and mode B, in which switching element 14 is turned off and the electromagnetic energy of transformer 10 is converted into secondary current Iout and then to the charging (electrostatic) energy of capacitor 20, are repeated alternately. Transition from mode A to mode B occurs when comparator 28 detects the fact that primary current Isw has increased to a prescribed value or larger. Transition from mode B to mode A occurs when comparator 30 detects the fact that the voltage between the two terminals of primary coil 24 has dropped to reference value VF2 or lower.
For the conventional charging circuit, however, in the following special cases, switching element 14 stays in the ON or OFF state. As a result, the charging operation virtually stops.
When the power supply capacity of battery 12 is reduced to near the empty state, as shown in FIG. 10, primary current Isw increases slowly since switching element 14 is turned on, and a fairly long time is required to reach the threshold value (VF1/R16). Switching element 14 stays in the ON state. During that period, the electromagnetic energy of transformer 10 is not converted into secondary current Iout, and the charging operation virtually stops.
When the charging voltage of capacitor 20 becomes abnormally low or becomes negative due to a short circuit or other reasons on the secondary side, as shown in FIG. 11, the induced electromotive force on the secondary side or secondary voltage VA generated by electromagnetic induction immediately after switching element 14 is turned off becomes small. This is reflected on the primary side. The induced electromotive force on the primary side also becomes small. In this case, when the switch terminal voltage VSW on the primary side does not exceed threshold value (VBAT+VF2), the output logic of comparator 30 is held without change. The signal to turn on switching element 14 again cannot be generated, and movement to the next charging cycle is not possible. The charging operation virtually stops.
When malfunction occurs in comparators 28, 30 or control logic 35 (RS latch circuit 34, one-shot circuit 38) due to electrostatic electricity, electromagnetic waves, or other external reasons, freezing may occur with switching element 14 in either the ON state or OFF state. The charging operation also stops in this case.
A general object of the present invention is to solve the problems of the conventional technology by providing a charging circuit that can continue the switching operation for charging in a stable and reliable manner, even under special conditions, so that the charging operation can be carried out reliably.